a. Field of the Invention
The invention relates to distance measuring instruments and more particularly to a non-contacting gauge for measuring the distance between a gauge head and planar surfaces and for measuring the thickness of a planar member.
b. Prior Art
In the field of non-contacting thickness gauges, a large segment of prior art gauges rely on reflection, scattering or transmission of electromagnetic or nuclear radiation or on eddy currents. Still other gauges rely on acoustic waves, particularly ultrasonic waves for pulse-echo measurements of thickness. One of the problems with prior art pulse-echo devices is that acoustic waves of ultrasonic frequency partially penetrate the object being measured, thereby creating an uncertainty regarding the location of the reflective plane.
By using longer acoustic wavelengths, such as audio wavelengths, the waves can no longer penetrate most materials, but then pulse-echo techniques cannot be used for measurement of small thicknesses and distances, on the order of a few millimeters to a few thousandths of a millimeter, because the distance being measured becomes a small fraction of a wavelength and cannot be accurately resolved. Previously, it was thought that to measure small distances, wavelengths smaller than the distance to be measured were preferred. An alternate gauge, not relying on any wave motion is the air gauge.
In an air gauge an air supply is usually connected to two air channels, one being a reference channel, in which a preset air leak is established from an orifice, the other being a measuring channel having another orifice which is inserted into an object, such as a pipe, at a slight distance therefrom. The rate of leakage in the measuring channel corresponds to a distance from the orifice to the object. Two such orifices, diametrically opposed in a plug are used to measure the inside diameter of bored holes. Certain air gauges of the prior art use the input air supply to operate a differential pressure meter so that differences between the reference channel and the measuring channel can be directly read by a gauge. Prior art air gauges are not limited to measuring inside diameters of pipes. U.S. Pat. No. 3,968,679 teaches a pneumatic sensor head for monitoring the thickness of a continuously moving sheet of material passing through the head. One of the problems with prior art air gauges is that the measuring head must usually be precision machined and a continuous gas supply must be provided, such as by an air tank. Such a requirement limits the portability of the instrument and requires plumbing normally associated with gas supplies, including a master pressure gauge and tubing. Another problem with air gauges is a characteristically short measuring range, typically 0.002 to 0.003 inches.
A more serious problem is that using compressed gases means that a compressor was used to provide the gas supply and very typically compressors introduce measurable quantities of hydrocarbon particles into the gas, particularly droplets of oil. These particles can contaminate surfaces which the gas is blown against. Besides introducing contaminant particles, a gas stream may agitate nearby particles and cause them to adhere to the surface of the object being measured.
The problem of contamination becomes acute in the manufacture of semiconductor integrated circuits. Such circuits are deposited on substrates of silicon whose thickness must be known. Yet because the silicon substrate will eventually contain hundreds of identical integrated circuits, the thickness measurement should be made by a non-contacting gauge which does not introduce any contaminants to the surface of the substrate. A single particle can cause the failure of an entire integrated circuit. Since the value of even one circuit can be substantial, it is important to guard against contaminants, especially particulates. The problem of surface contamination is not unique to integrated circuit manufacture. Other substances and substrates, such as optical materials and solid objects having planar surfaces must often be measured with a non-contacting gauge which does not introduce contaminants to the surface.